Multi-element jet engine noise suppressor nozzle systems have presented considerable design problems in the past. They have been difficult to fabricate and to stow away during cruise flight. Their overall girth, length, and base area have in some cases restricted takeoff roll attitudes and have produced aerodynamic drag penalties. Their fixed nozzle discharge areas do not benefit some engines which require a variable nozzle area to accommodate a change in the engine power setting. Further, multi-element jet noise suppressors have tended to be ineffective at lower jet velocities common with the lower power settings for turbojet engines as well as the entire range of power settings for high bypass ratio engines.
A jet noise suppression device for large supersonic airplanes has several unique requirements which must be satisfied. The first is to provide a low drag pod, and this normally necessitates that nothing protrudes or increases the diameter of the aft cowl portion beyond that diameter which has been selected for optimum thrust-minus-drag.
The second requirement is to provide structure having reasonably small drag penalties at subsonic cruise and takeoff conditions. The nozzle functions must provide a thrust reverser with adequate flow area so that the exhaust gas can be directed to selected areas where reingestion is minimized. Only about one half of the perimeter of the exhaust system is usable because of this requirement.
The third is that a high level of jet noise suppression is required for large supersonic airplanes at takeoff in order to meet the future FAA noise rule goals.
In the prior art, internal ventilation for jet noise control has been attempted. Crossover tubes, similar to those disclosed in U.S. Pat. No. 3,779,282, have been used to provide internal ventilation but they become less practical as the ventilation requirement increases. In a typical large supersonic airplane, for example, the ventilation jet area must be about equal to the main jet area in order to achieve the necessary large noise suppression level. A typical exhaust system for a large supersonic aircraft, such as the SST, can theoretically provide this flow area if the blockage of the supply and collection ducts are close to zero. This is practically impossible to achieve with crossover tubes because a supply and collection annulus must be integrated with the tubes and this uses up a large part of the available area. From the known prior art, it appears that the solution of the problem of maximizing the amount of ventilation air has not been attempted in the above aircraft engines.
A search of the patent literature illustrates a number of systems in which ambient air has been introduced into a jet engine nozzle so as to flow adjacent the central streamlined body in the nozzle. U.S. Pat. No. 3,432,100 discloses such a system in which ventilating air is fed into the nozzle through opening flaps 12 so as to converge with an inner exhaust driving stream and flow along a central streamlined body.
U.S. Pat. No. 3,637,041 discloses a fanjet engine having a noise suppressor in the tail pipe. There are partitions spaced around the periphery of an inner wall of a shroud and they extend radially inwardly to divide the engine discharge zone into two sets of alternately interposed flow passages. One set of vanes in the flow passages deflects fan air inwardly into the turbine exhaust gas flow and a second set of vanes deflects the turbine exhuast gas outwardly into the fan airflow, so as to mix the exhaust gas and the air to reduce the noise level.
U.S. Pat. No. 3,910,375 illustrates a jet engine silencer in which ambient air is permitted to flow from the exterior of the nozzle into a central body. The entry direction is such to cause substantial loss in the volume of flow due to friction and flow turbulence. The air is caused to mix with the exhaust gases externally of the nozzle. This arrangement provides for only a small internal ventilation area.
U.S. Pat. No. 3,779,282 provides a flow control system for inverting the positions of two adjacent constant air flow patterns. Such a device is adapted to be used in a jet engine annulus in which the total flow may divided into two separate equal or unequal constant area duct means having a combined total cross-sectional area substantially equal to the cross-sectional area of the original passageway in the form of an annulus. The flow patterns are formed in annular first and second ducts divided into duct elements shaped and arranged such that two annular flow patterns are discharged at the exit end of the passageway in an inverted relationship to that which exists at the entrance end of the passageway. U.S. Pat. Nos. 4,085,583; 3,792,584; 3,854,286; and 3,938,328 illustrate uses of the device disclosed in U.S. Pat. No. 3,779,282.
The following additional patents, found in the search, disclose noise suppressors for jet engines:
U.S. Pat. No. 3,032,981 PA1 U.S. Pat. No. 3,262,264 PA1 U.S. Pat. No. 3,333,772 PA1 U.S. Pat. No. 3,493,178 PA1 U.S. Pat. No. 3,578,106 PA1 U.S. Pat. No. 3,695,387 PA1 U.S. Pat. No. 4,026,472 PA1 U.S. Pat. No. 4,054,030